A surgeon performing an invasive procedure on the human brain requires a method of determining a trajectory through the brain that avoids large blood vessels and critical function areas. Such a trajectory could be determined from a three-dimensional representation of the brain that clearly distinguished among stationary tissue types and between stationary tissue and vascular surfaces. This three dimensional representation could be used, for example, with stereotaxic procedures performed through the intact skull or interstitial laser surgery in which the needle or optical fiber, once inserted, is hidden deep with in the tissue and cannot be visualized.
Procedures associated with the knee, spine, and other internal body structures might also benefit from such a "segmented" three dimensional representation.
The location of blood vessels within an internal body structure may be identified in x-ray procedures such as radiographic angiography and computed tomography by injection of a contrast agent into these vessels. However, x-rays generally differentiate poorly between soft tissue types and thus would be unsuitable for distinguishing between stationary tissues associated, for example, with functional areas of the brain. Also, some patients have an adverse reaction to the contrast agent needed to image blood with x-ray techniques. A preferable technique, therefore, is to employ magnetic resonance imaging (MRI) both to generate a three dimensional representation of the vascular surfaces and of the surfaces of the various stationary tissue types.
A. Acquisition of MRI data
As is understood in the art, three-dimensional arrays of data representing one or more physical properties of a body at regular grid positions may be obtained through the use of MRI.
In an MRI system, the body to be imaged is placed in a strong polarizing magnetic field. Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the gyromagnetic constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.O), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (RF excitation pulse) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M.sub.z is tipped, and hence the magnitude of the net transverse magnetic moment M.sub.t depends primarily on the length of time and the magnitude of the applied excitation pulse.
The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation pulse is terminated. The characteristics of this resonance signal, for example, the decay constants of the signal T.sub.1 and T.sub.2 representing the loss in the spins of longitudinal magnetization and phase coherence, respectively, are related to the physical properties of the tissue containing the excited spins and may be measured to reveal information about the tissue type.
Construction of an image from the resonance signal requires that the components of the resonance signal from spatially separated spins be distinguished and located. Typically this is done by applying a sequence of orthogonal magnetic gradients to the spins prior to and while recording the resonance signal. These gradients cause the spins at various locations to precess at different resonant frequencies depending on their position along the gradient axis. Thus, the position and signal component of spins at each location may be isolated and identified.
An image intensity value is associated with the characteristics of each isolated component signal, and together with the identified position, used to construct an image according to well known reconstruction techniques. Each intensity value and position define a picture element (pixel) of the resultant image.
The prior art is replete with excitation and gradient sequences that are designed to take advantage of particular magnetic resonance phenomena to advantageously differentiate tissue types. Each such pulse sequence and the acquisition of a resulting resonance signal is termed a "measurement cycle".
One such sequence is the well known "spin echo" sequence (SE). In the spin echo sequence, a "180.degree. RF" pulse is applied some time after the RF excitation pulse to flip the magnetization of the sample of precessing nuclei approximately 180.degree.. To the extent that the individual nuclei have dephased after the RF excitation pulse because of magnetic field inhomogeneities (T.sub.2 '), the 180.degree. RF pulse reverses the accumulated phase shifts and causes these nuclei to begin rephasing. At time (TE/2) after the 180.degree. RF pulse equal to the delay between the RF excitation pulse and 180.degree. pulses, the nuclei are in phase and produce a "spin echo". The amplitude of this spin echo is less than the initial amplitude of the free induction decay (FID) as a result of T.sub.2 dephasing. This T.sub.2 decay is not reversed by the 180.degree. RF pulse. Hence the relative amplitude of two or more spin echoes may be used to directly derive T.sub.2 without contribution from T.sub.2 ' . Generally, the amplitude of the spin echo from a given volume element of the imaged object is: EQU S.sub.1 =Pe.sup.-TE/T2 (l-e.sup.-TE/T2) (1)
where P is proportional to the density of the spins in that volume element.
The spin echo pulse sequence is described generally in "Magnetic Resonance Imaging, Principles and Applications" by D. N. Kean and M. A. Smith and the references therein cited.
A second imaging sequence is the "gradient recalled echo" sequence, (GRE). In the gradient recalled echo sequence, a negative gradient is used to dephase the spins which are then rephased by a positive gradient to produce a gradient echo signal. The amplitude of the gradient echo from a given volume element of the imaged object is: ##EQU1## where .alpha. is the "flip angle" of the spins from the z-axis as produced by the 180.degree. RF pulse.
In addition to the use of MRI to collect information concerning the static physical properties of tissue, there are a number of well known MRI techniques for measuring the motion, or flow of fluids, such as blood, within a region of interest.
One such method is the "time-of-flight" method in which a bolus of spins is excited as it flows past a specific upstream location and the state of the resulting resonance signal is examined at a downstream location to determine the velocity of the bolus. This method has been used for many years to measure flow in pipes, and in more recent years it has been used to measure blood flow in human limbs. Examples of this method are disclosed in U.S. Pat. Nos. 3,559,044; 3,191,119; 3,419,793; and 4,777,957. The use of the above described GRE sequences to make time-of-flight measurements is well known.
A subclass of time-of-flight methods is comprised of "inflow/outflow methods" in which the spins in a single, localized volume or slice are excited and the change in the resulting resonance signal is examined a short time later to measure the effects of excited spins that have flowed out of the volume or slice and the effects of differently excited spins that have flowed in to the volume or slice. Examples of this method are described in U.S. Pat. Nos. 4,574,239; 4,532,473; and 4,516,582, as well as in "Three-Dimensional Time-of-Flight Magnetic Resonance Angiography Using Spin Saturation" by C. L. Dumoulin et al. published in Magnetic Resonance In Medicine 11:35-46 (1989).
A third measurement technique is the "phase contrast method" which relies upon the fact that a resonance signal produced by spins flowing along a magnetic field gradient exhibits a phase shift which is proportional to the velocity of the spins. Two signals are acquired, one to establish a reference and one to determine the phase shift proportional to velocity along the particular gradient. Where the direction of flow is known, or the flow component in only one direction is desired, two MRI measurement cycles are sufficient. Otherwise the sequence is repeated for each gradient for a total of six acquisitions to determine the flow along each of the three orthogonal gradient axes. This method is referred to as the six-point method and is described in commonly assigned U.S. Pat. No. 4,918,386, issued Apr. 17, 1990 and incorporated herein by reference.
An improvement in this phase contrast method, which recognizes that the reference signal may be shared for all three of the flow measurements along gradient axes, is described in commonly assigned application Ser. No. 07/564,945, now U.S. Pat. No. 5,093,620, filed Aug. 9, 1990 and entitled "ENCODING FOR MRI PHASE CONTRAST MEASUREMENT" and incorporated herein by reference. This method is termed the "balanced four point method" and provides shorter acquisition time and improved signal to noise ratio.
B. Segregation of tissue types
It is known to use MRI data to distinguish flowing blood from stationary tissue by means of the flow imaging techniques as described above Further, two or more conventional MRI sequences such as the spin echo sequence described above may be used to distinguish or contrast various types of stationary tissue. For Example, commonly assigned patent application Ser. No. 07/466,526 now still pending filed Jan. 17, 1990 and entitled: "SYSTEM AND METHOD FOR SEGMENTING INTERNAL STRUCTURES CONTAINED WITHIN THE INTERIOR REGION OF A SOLID OBJECT" and incorporated herein by reference, describes a method of discriminating between stationary tissue types through the use of two conventional spin echo MRI imaging sequences.
As recognized in the above referenced application, the use of a simple threshold for differentiating between stationary tissue types based on the data from a single MRI sequence will not be, in general, successful. Multiple stationary tissue types will be within any given range established by that threshold.
This problem may be overcome by using the data from each of two spin echo sequences adjusted to measure contrasting characteristics of the stationary tissue. The two sets of data are then used to generate a two-dimensional scatter plot in which the abscissa and ordinates of the points on the scatter plot are determined by the values of corresponding data in each of the two data sets.
Representative tissue types are identified to certain of the points on the scatter plot and the surface of the scatter plot is partitioned according to calculated probability distributions based on the statistics of these few points. This partitioning of the scatter plot generates a features map which may be used to classify the remaining data of the two sets of data acquired.
Separating stationary tissue types and differentiating vascular surfaces according to this technique requires three separate imaging sequences: two conventional MRI sequences to provide differentiation between stationary tissue types by use of the scatter plot and features map and one flow imaging sequence to clearly delineate vascular surfaces. The need to use three sequences with widely differing imaging parameters makes this method undesirably time consuming.